Due to aging population, the elderly population would be expected to increase every year, and more persons would have disability in walking or muscle weakness or joint problem. Stroke is a cerebrovascular accident that often occurs among the elderly population. Post-stroke survivors usually suffer from hemiplegia with severe weakness and loss of motor control over one side of the body, which significantly reduces their mobility and functional independency. Independent walking is an important function for good quality of life. However, weakness or impairment of the ankle muscles could significantly alter the gait pattern and impair the lower limb functionality. Thus, gait recovery is usually the first priority in stroke rehabilitation.
Drop foot is a common gait abnormality at the ankle joint that is caused by nerve injuries or neurological disorder such as stroke or multiple sclerosis. Patients would show muscle weakness at the dorsiflexors, and would often come with spasticity at the opposite plantar flexors. Abnormal gait pattern would increase energy expenditure and reduce balance ability in walking. Patient would be more prone to falling, tiredness, or joint limb pain.
Normal gait is a cyclic pattern that involves interaction between different lower limb body segments and joints working together to shift the center of mass of the ambulator forward step-by-step. During the normal gait, the ankle joint plays an important role in all aspect of locomotion, including motion control, shock absorption, stance stability, energy conservation, and propulsion. A gait cycle consists of two main parts: the stance phase and the swing phase. It begins with initial contact. The controlled loading response at heel strike absorbs shock and decelerates the foot before the foot completely landed. It provides gait stability during stance phase, when the ankle movement translates the body center of mass forward. At terminal stance, the ankle releases energy that stored in tendon and ligament of plantar flexor to provide propulsive force for push-off. In swing phase, the ankle dorsiflexor contracts to lift up the forefoot. It provides foot clearance and prevents the foot from stumbling into obstacles on the ground.
During walking of drop foot patients, the weak dorsiflexors fail to offer controlled loading response and shock absorption after heel strike. It leads to audible foot slap at initial contact. Weak plantar flexors and unstable ankle foot complex reduces stability during stance phase. Shorter stance phase and reduced propulsive power at push-off results in slower walking speed. In swing phase, the ankle cannot provide enough dorsiflexion moment to lift up the foot, so the foot is dragged on the ground. Stepping gait, hip hiking, and outward leg swinging are compensation mechanisms of drop foot. Patients tend to increase the knee and hip flexion during swing phase to allow more foot clearance. Ironically stroke survivors often have weak knee muscles. The result is an unstable and ineffective gait.
Effective gait training that achieves good therapeutic effect must satisfy these requirements: high intensity, high repeatability, long duration, and large task variation. It is encouraged that stroke patients to do more walking, especially in a outdoor setting where walking conditions may vary much. There are some gait assistive devices, such as U.S. Pat. No. 7,628,766 “Lower extremity enhancer” and U.S. Pat. No. 8,591,438 “Walk assisting device which defines a rigidity of portions thereof”. They are targeted at providing bilateral gait assistance that is more suitable for completely paralyzed spinal cord injury patients, while stroke survivors who only need unilateral gait assistance are not served.
Gait rehabilitation usually would prescribe an ankle foot orthosis (AFO) to the stroke survivors. There are three types of AFO: passive, semi-active, and active. Passive AFO can be used to support the drop foot during swing phase. They are often rigid plastic that limits the ankle joint at the neutral 90 degrees to prevent plantar flexion. However, this kind of AFO imposes undesirable restriction to the ankle joint movement, thus unnatural gait. There are some articulated AFO with spring and dampers (U.S. Pat. No. 2,516,872A “Ankle brace”, U.S. Pat. No. 3,064,644A “Lower leg brace”, U.S. Pat. No. 5,176,623A “Multiple fixed angle orthopaedic appliance”, U.S. Pat. No. 6,171,272B1 “Short leg brace”, U.S. Pat. No. 6,350,246B1 “Ankle and foot therapeutic device”, U.S. Pat. No. 7,691,076 “Articulated custom ankle-foot orthosis systems”, U.S. Pat. No. 8,382,694 “Ankle-foot orthotic for treatment of foot drop”, and US patent application no. 2006/0084899 “Hinged ankle brace”). Although they can control ankle joint movement without excessive restriction, they offer neither active assistance nor adaptation to different walking conditions. Semi-active AFO contains sensory feedback to determine different walking tasks, and then the joint impedance can be controlled using adjustable springs or dampers to support weak ankle dorsiflexion. However, these semi-active AFO cannot generate torque for active ankle joint assistance, especially at push-off. The power source comes from the wearer's gait but does not add energy to the system.
Existing active AFO have different types of motors and power transmission mechanisms. US patent application no. 2011/0112447A1 “Portable active fluid powered ankle-foot orthosis” is an active robot that uses pneumatic power to actuate the ankle joint. But the fluid container added weight to another body part and is bulky. U.S. Pat. No. 8,771,211 “Ankle orthosis” uses cam drive assembly and US patent application no. 2005/0059908 “Apparatus for assisting body movement” uses contraction for power transmission. U.S. Pat. No. 8,808,214 “Active ankle foot orthosis” is an articulated AFO with adjustable joint impedance. These exoskeleton robots can be programmed to moderate drop foot walking according to gait phase, but they are too cumbersome to wear under clothes with unmodified footwear. Tethered power source also limits the device in clinical or laboratory utilities.